Advances in integrated circuit architecture and miniaturization have resulted in greater numbers of functions being encompassed on single chips, necessitating the use of chips of larger dimensions, increased lead density and/or reduced lead pitch. The increasing use of fine pitch integrated circuit devices (FPD), which presently have lead pitches on the order of 0.025 inches, and the projected use of FPDs having lead pitches on the order of 0.008 inches, has taxed the placement capabilities of existing integrated circuit placement systems.
The continuing reduction in lead pitch dimensions has a corresponding effect on placement error tolerances. Placement error is a function of angular error, translational error, and geometric differences between the FPDs and the circuit board. Transnational error is the X, Y offset of an FPD from a specified circuit board placement site and is primarily related to lead pitch. Angular error is the rotational difference between the specified circuit board placement site and the actual site and is related to both lead pitch and the size of the FPD.
For rotation of a FPD about its center, there is an apparent offset of the leads relative to the board pads, with the misalignment being most severe at the corners. For example, rotation of a 1.5 inch FPD about 0.15.degree. with respect to its center causes a lateral offset of about 2 mils for a corner lead. Misalignment problems in FPDs, in conjunction with lead-to-pad overlap requirements, place stringent requirements on component placement systems.
A conventional means of component placement and alignment is the "ballistic" placement system exemplarily illustrated in FIGS. 1A-1C which comprises a vision subsystem that includes two cameras C1, C2 and a mechanical positioning subsystem MPS that provides for X, Y positioning of a movable placement head MPH and Z, .theta. movement of a component tool CT mounted on the placement head. In the ballistic system, a component is retrieved by moving the placement head MPH to a feed location, picking up the component, and analyzing the component image in the vision subsystem utilizing the field-of-view of a first camera C1 as illustrated in FIG. 1A. The location and magnification of the first camera C1, the position of the placement head MPH, and the location of the component in field-of view of the vision subsystem are factors determinative of the position of the component with respect to a datum point of the system.
The placement head MPH is then moved to the target site on the circuit board and the second camera C2 is used to make measurements on either the board mounting pads or fiducial references on the board as illustrated in FIG. 1B. Determination of the board position is also subject to the foregoing factors. Translational and rotational corrections are calculated based upon board and component positions, and these corrections are applied by the system in placing the component on the board as illustrated in FIG. 1C.
There are several inherent limitations in ballistic systems that make them less effective for FPD placement and alignment. Component mounting is based upon the combined absolute accuracy of the vision subsystem and the mechanical positioning subsystem throughout the working envelope of the system. Vision is not utilized for final lead-to-pad alignment and placement. Ensuring proper lead-to-pad alignment with the accuracy demanded by IC devices such as FPDs requires simultaneous observation of the lead-to-pad alignment on each of the four sides of the FPD during placement operations. The conventional ballistic placement system described hereinabove and illustrated in FIGS. 1A-1C has a configuration that does not facilitate utilization of the vision subsystem during the actual placement operations.
In addition, the magnification of both cameras must be known and maintained with great accuracy in a ballistic placement system. The location of the cameras on the system must be known with a high degree of accuracy. If fiducials are utilized as board references, the relationship between fiducials and the board mounting pads must be specified and maintained on a site to site basis.
The mechanical positioning subsystem must be capable of accurate movement to position the workpieces with respect to the vision subsystem and with respect to each other. Generally, achievement of the requisite degree of absolute movement accuracy in a ballistic type placement system requires precision components and/or frequent calibration, both options increasing the expense of such a system.
The impact of these limitations may be minimized by design and calibration, but with a corresponding cost impact. Minimizing these limitations by design results in disproportionately higher equipment costs, and correction by calibration entails increased operating costs and inconvenience.
Traditional ballistic placement systems, even with vision enhancement, generally do not achieve the accuracy required for the placement and alignment of FPDs on printed circuit boards. In addition, traditional ballistic placement systems generally do not have an inherent capability, and are not easily modified, for on-head soldering or soldering-in-place to secure aligned FPDs to printed circuit boards.
Typically, bonding may be accomplished by any of several methods such as reflow soldering. The board may be transported off the placement system for bonding such as to a mass reflow machine. Alternatively, selective reflow techniques may be utilized to reflow solder the component prior to transporting the board off the placement system. Or, the component may be reflow bonded at time of placement with equipment mounted integral to the system.
For FPDs, the third alternative is generally the most advantageous. As discussed above, misalignment is a critical consideration in placing and aligning FPDs on circuit boards. Misalignment may occur when a component is released after being placed and aligned, and/or when the circuit board is subjected to further handling. Reflowing to bond the component to the circuit board while the component is being held in place by the placement apparatus significantly reduces or eliminates lead-to-pad misalignment problems. However, the placement head configuration of typical ballistic placement systems is not conducive to the integration and/or utilization of an integral heater array subsystem.